BACKGROUND Blunted blood pressure (BP) dipping is an established predictor of adverse cardiovascular outcomes. Although blunted BP dipping is more common in African Americans than whites, the factors contributing to this ethnic difference are not well understood. This study examined the relationships of BP dipping to ethnicity, body mass index (BMI), sleep quality, and fall in sympathetic nervous system (SNS) activity during the sleep-period. METHODS On three occasions, 128 participants with untreated high clinic BP (130-159/85-99 mm Hg) underwent assessments of 24-h ambulatory BP (ABP), sleep quality, (evaluated by sleep interview, self-report, actigraphy) and sleep-period fall in sympathetic activity (measured by waking/sleep urinary catecholamine excretion). RESULTS Compared to whites (n = 72), African Americans (n = 56) exhibited higher sleep-period systolic (SBP) (P = 0.01) and diastolic BP (DBP) (P < 0.001), blunted SBP dipping (P = 0.01), greater BMI (P = 0.049), and poorer sleep quality (P = 0.02). SBP dipping was correlated with BMI (r = -0.32, P < 0.001), sleep quality (r = 0.30, P < 0.001), and sleep-period fall in sympathetic activity (r = 0.30, P < 0.001). Multiple regression analyses indicated that these three factors were independent determinants of sleep-period SBP dipping; ethnic differences in dipping were attenuated when controlling for these factors. CONCLUSIONS Blunted BP dipping was related to higher BMI, poorer sleep quality, and a lesser decline in sleep-period SNS activity. Although African-American ethnicity also was associated with blunted dipping compared to whites in unadjusted analyses, this ethnic difference was diminished when BMI, sleep quality, and sympathetic activity were taken into account.